CODESYS example: Modbus TCP RTU Slave | Industrial Raspberry Pi

In this example, we configure and run both a Modbus TCP slave and a Modbus RTU slave on the RevPi Connect+ with CODESYS Control for Linux ARM/ARM64. The application is designed to copy the first 10 holding registers to the 10 input registers of both slaves.

Prerequisites #

Hardware #

✓ RevPi Connect+
✓ One Modbus TCP slave
✓ One Modbus RTU slave

For detailed instructions about how to set up your system, see Getting Started.

Software #

✓ CODESYS Development System installed on your PC

To ensure a compatible system with suitable software, see Software Compatibility.

Overview #

The following figure provides an overview of the CODESYS device tree for the setup:

Adapting for RevPi Core or RevPi Compact

▷ Open the context menu of RevPi Connect in the CODESYS device tree.

▷ Select Update device.

▷ Choose RevPi Core or RevPi Compact as the replacement and rename the device accordingly.

Step 1: Configuring Modbus RTU Serial Port #

For RevPi Connect or RevPi Core

▷ Edit the configuration file:

sudo nano /etc/CODESYSControl_User

▷ Add the following line:

[SysCom]
Linux.Devicefile=/dev/ttyUSB

▷ Map the COM ports in CODESYS as follows:

/dev/ttyUSB0 → COM port 1
/dev/ttyUSB1 → COM port 2
/dev/ttyUSBn → COM port n

If an extra USB-485 converter is connected, the correct COM port has to be identified.

For RevPi Compact:

▷ Edit /etc/CODESYSControl_User for the default RS485 Port and add:

   [SysCom]
   Linux.Devicefile=/dev/ttyAMA

▷ Add the following line instead for using USB-485 converter:

   [SysCom]
   Linux.Devicefile=/dev/ttyUSB

Step2: Configuring Modbus RTU Slave #

▷ Map Holding Registers and Input Registers to the application POU_Modbus to make the data accessible to the application logic.

Step 3: Configuring Modbus TCP Slave #

The configuration for the Modbus TCP slave is similar to that for the Modbus RTU slave.

Step 4: Simulating Modbus TCP and RTU Masters #

▷ Use the application QModMaster for simulation.
You can download it here.

Write Multiple Registers: Writes data to the holding registers of the slave.

Read Holding Registers: Reads back the input register data of the Modbus TCP/RTU slave running on the RevPi device.

Step 5: Modbus TCP Unit ID #

The Modbus TCP Unit ID serves the same function as the slave ID.
The Unit ID 255 is recommended for addressing the gateway itself.
When using QModMaster, note that it uses Unit ID 0 instead of 255 for addressing the gateway.